Node for an optical network

ABSTRACT

An optical network including an input to receive from an optical network light comprising plural wavelength components. An optical wavelength selective filter, optically connected to the input, extracts a first wavelength component of the plural wavelength components from the light, thereby providing a first optical signal including the first wavelength component and a second optical signal including a remainder of the plural wavelength components a light emitter to provide a modulated broadband optical signal. A first output, optically connected to the optical wavelength selective filter, receives a first portion of the second optical signal for transmission to a light detector and a second output, optically connected to optical wavelength selective filter, receives a second portion of the second optical signal for transmission to the optical network.

TECHNICAL FIELD

The present invention relates to nodes for optical networks, systemscomprising such nodes, optical networks comprising such nodes, and tomethods of operating the same.

BACKGROUND

In many communications applications, such as avionics networks, it isdesirable to use optical signals to communicate signals between nodes ofthe network. Optical communications may provide a lighter alternative toelectronic communications, may provide higher resistance toelectromagnetic interference (EMI) and background noise, and may providehigher communication speeds.

Some communications networks may use wavelength division multiplexing(WDM) whereby different nodes of the network are allocated differentwavelengths at which they transmit optical signals. In someapplications, different nodes may transmit signals at differentfrequencies along the same optical communications path, such as a commonoptical fibre.

In some critical applications, such as aircraft avionics networks, eachtype of node that is to be used in the network must pass rigoroustesting. Such testing may involve significant costs and so it may bedesirable to reduce the number of types of node that must undergo suchtesting.

Furthermore, in some network topologies it is desirable for signalstransmitted by a node to be removed from the network when the signalsare no longer required. Such removal can be computationally intensiveand may require active optical devices that can be complex and costly totest, and may lead to latency as data is read and retransmitted from onenode to another.

SUMMARY

A first aspect of the present invention provides a node for an opticalnetwork, the node comprising: an input to receive from an opticalnetwork light comprising plural wavelength components; an opticalwavelength selective filter, optically connected to the input, toextract a first wavelength component of the plural wavelength componentsfrom the light, thereby providing a first optical signal comprising thefirst wavelength component and a second optical signal comprising aremainder of the plural wavelength components; a first output opticallyconnected to the optical wavelength selective filter to receive a firstportion of the second optical signal for transmission to a lightdetector; and a second output optically connected to optical wavelengthselective filter to receive a second portion of the second opticalsignal for transmission to the optical network.

Optionally, the node comprises one or more optical connectors to connectthe node to an avionics unit, the one or more optical connectorscomprising: a first optical interface, optically connected to the secondoutput, to receive an optical signal from the avionics unit; a secondoptical interface, optically connected to the first output, to transmitthe first portion of the second optical signal to the avionics unit; anda third optical interface, optically connected to the optical wavelengthselective filter, to transmit the first optical signal to the avionicsunit.

Optionally, the node comprises a first optical circulator, a secondoptical circulator, and a beam splitter, wherein the first and secondoptical circulators each comprise a first port, a second port, and athird port, and the first optical wavelength selective filter comprisesa first port and a second port, wherein: the first port of the firstoptical circulator is optically connected to the first opticalinterface, the second port of the first optical circulator is opticallyconnected to the first port of the first optical wavelength selectivefilter, and the third port of the first optical circulator is opticallyconnected to the output via the beam splitter; the first port of thesecond optical circulator is optically connected to the input, thesecond port of the second optical circulator is optically connected tothe second port of the first optical wavelength selective filter, andthe third port of the second optical circulator is optically connectedto the third optical interface; wherein the beam splitter is arranged toreceive light from the third port of the first optical circulator andtransmit light to the output and to the second optical interface.

Optionally, the node comprises a first optical circulator, a secondoptical circulator, a third optical circulator and a beam splitter,wherein the first and third optical circulators each comprise a firstport, a second port, and a third port, the second optical circulatorcomprises a first port, a second port, a third port and a fourth port,and the first optical wavelength selective filter comprises a first portand a second port, wherein: the first port of the first opticalcirculator is optically connected to the first optical interface, thesecond port of the first optical circulator is optically connected tothe first port of the first optical wavelength selective filter, and thethird port of the first optical circulator is optically connected to thefirst port of the second optical circulator; the second port of thesecond optical circulator is optically connected to the first port ofthe third optical circulator, the third port of the second opticalcirculator is optically connected to optically connected to the secondport of the first optical wavelength selective filter, and the fourthport of the second optical circulator is optically connected to thethird optical interface; the second port of the third optical circulatoris optically connected to the output via the beam splitter and the thirdport of the third optical circulator is optically connected to theinput; wherein the beam splitter is arranged to receive light from thesecond port of the third optical circulator and transmit light to theoutput and to the second optical interface.

Optionally, the second optical circulator is optically connected to thethird optical circulator via a removable connector.

Optionally, the removable connector is an expanded beam connector.

Optionally, the node comprises a first optical circulator, a secondoptical circulator, a third optical circulator and a beam splitter,wherein the first and third optical circulators each comprise a firstport, a second port, and a third port, the second optical circulatorcomprises a first port, a second port, a third port and a fourth port,and the optical wavelength selective filter comprises a first port and asecond port, wherein: the first port of the first optical circulator isoptically connected to the first optical interface, the second port ofthe first optical circulator is optically connected to the first port ofthe first optical wavelength selective filter, and the third port of thefirst optical circulator is optically connected to the first port of thesecond optical circulator via the beam splitter; the second port of thesecond optical circulator is optically connected to the first port ofthe third optical circulator, the third port of the second opticalcirculator is optically connected to optically connected to the secondport of the first optical wavelength selective filter, and the fourthport of the second optical circulator is optically connected to thethird optical interface; the second port of the third optical circulatoris optically connected to the output and the third port of the thirdoptical circulator is optically connected to the input; wherein the beamsplitter is arranged to receive light from the third port of the firstoptical circulator and transmit light to the first port of the secondoptical circulator and to the second optical interface.

Optionally, the node comprises a first optical circulator, a secondoptical circulator, a first beam splitter and a second beam splitter,wherein the first and third optical circulators each comprise a firstport, a second port, and a third port, the second optical circulatorcomprises a first port, a second port, a third port and a fourth port,and the optical wavelength selective filter comprises a first port and asecond port, wherein:

the first port of the first optical circulator is optically connected tothe input, the second port of the first optical circulator is opticallyconnected to the first port of the second optical circulator, and thethird port of the first optical circulator is optically connected tooutput via the first and second beam splitters; the second port of thesecond optical circulator is optically connected to the first port ofthe first optical wavelength selective filter, the second port of thesecond optical circulator is optically connected to the third opticalinterface, and the fourth port of the second optical circulator isoptically connected to the second port of the first optical wavelengthselective filter; the first beam splitter is arranged to receive lightfrom the third port of the first optical circulator and to transmitlight to the second optical interface and to the second beam splitter;and the second beam splitter is arranged to receive light from the firstoptical interface and from the first beam splitter and to transmit lightto the output.

Optionally, the first optical circulator is optically connected to thesecond optical circulator via a removable connector.

Optionally, the removable connector is an expanded beam connector.

Optionally, the node comprises a third optical circulator and a fourthoptical interface arranged to receive a removable second opticalwavelength selective filter, the third optical circulator comprising afirst port, a second port and a third port, wherein: the first port ofthe third optical circulator is optically connected to the first opticalinterface, the second port of the third optical circulator is opticallyconnected to the fourth optical interface, and the third port of thethird optical circulator is optically connected to second beam splitter.

Optionally, the fourth optical interface comprises an expanded beamconnector.

Optionally, the node comprises the second optical wavelength selectivefilter, the second optical wavelength selective filter having a samewavelength selectivity substantially matching a wavelength selectivityof the first optical wavelength selective filter.

Optionally, the second optical wavelength selective filter comprises afibre Bragg grating.

Optionally, one or more of the first, second and third opticalinterfaces comprises an expanded beam connector.

Optionally, the node comprises an avionics unit, the avionics unitcomprising: a light emitter to provide a modulated broadband opticalsignal to the first optical interface; and a first light detector toreceive light from the second optical interface.

Optionally, the avionics unit comprises a second light detector arrangedto receive light from the third optical interface.

Optionally, the node comprises a non-reflective termination opticallyconnected to the third optical interface.

Optionally, the first optical wavelength selective filter comprises afibre Bragg grating.

A second aspect of the present invention provides a node for an opticalnetwork, the node comprising: an output to transmit to an opticalnetwork a first optical signal comprising a first wavelength component;an input to receive, from the optical network, light comprising pluralwavelength components; and an optical interface optically connected tothe input and the output, the first optical interface being to receive aremovable optical wavelength selective filter arranged to extract thefirst wavelength component of the plural wavelength components from thelight, thereby providing an optical signal comprising the firstwavelength component and a second optical signal comprising a remainderof the plural wavelength components.

A third aspect of the present invention provides a system, comprising: anode according to the second aspect of the present invention; and aremovable optical wavelength selective filter to optically connect tothe optical interface of the node, the optical wavelength selectivefilter being to extract the first wavelength component of the pluralwavelength components from the light.

Optionally, the optical wavelength selective filter is opticallyconnected to the optical interface of the node.

Optionally, the removable optical wavelength selective filter comprisesa fibre Bragg grating.

Optionally, the removable optical wavelength selective filter comprisesa non-reflective termination.

A fourth aspect of the present invention provides an optical networkcomprising a node according to the first or second aspect of the presentinvention, or comprising a system according to the third aspect of thepresent invention.

Optionally, the optical network comprises a plurality of the nodes in aring network architecture.

A fifth aspect of the present invention provides an optical networkcomprising plural nodes, each of the plural nodes being arranged totransmit an optical signal at a distinct wavelength, wherein a node ofthe plural nodes is arranged to: transmit a first optical signalcomprising a first wavelength component; receive light comprising pluralwavelength components including the first wavelength component; extractthe first wavelength component from the light, thereby providing asecond optical signal comprising a remainder of the plural wavelengthcomponents; and transmit the second optical signal.

A sixth aspect of the present invention provides a vehicle comprising anode according to the first or second aspect of the present invention,or comprising a system according to the third aspect of the presentinvention, or comprising an optical network according to the fourth orfifth aspect of the present invention.

Optionally, the vehicle is an aircraft.

A seventh aspect of the present invention provides a method of operatinga node according to the first aspect of the present invention, or asystem according to the third aspect of the present invention, or anoptical network according to the fourth or fifth aspect of the presentinvention, the method comprising: transmitting an optical signalcomprising the first wavelength component from the node; receiving thelight comprising plural wavelength components including the firstwavelength component at the node; and extracting the first wavelengthcomponent from the light at the node, thereby providing the secondoptical signal comprising the remainder of the plural wavelengthcomponents.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described, by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a schematic diagram illustrating an example of a node for anoptical network of an embodiment of the invention;

FIG. 2 is a schematic diagram illustrating an example of a node for anoptical network of an embodiment of the invention;

FIG. 3 is a schematic diagram illustrating an example of a node for anoptical network of an embodiment of the invention;

FIG. 4 is a schematic diagram illustrating an example of a node for anoptical network of an embodiment of the invention;

FIG. 5 is a schematic diagram illustrating an example of an opticalnetwork of an embodiment of the invention;

FIG. 6 is a flow diagram illustrating a method of operating an avionicsnetwork of an embodiment of the invention; and

FIG. 7 is a schematic diagram illustrating an example of a vehicle of anembodiment of the invention.

DETAILED DESCRIPTION

FIG. 1 is a schematic diagram illustrating an example of a node 100 foran optical network according to an embodiment of the invention. The node100 is arranged to transmit an optical signal to and receive opticalsignals from other nodes in an optical network, such as an opticalintegrated avionics network. It will be understood that components shownin FIG. 1 are shown symbolically and as such the symbols used do notnecessarily reflect the structure of the components they represent.

The node 100 comprises an output 102 and an input 104.

The node 100 is arranged to transmit light via the output 102. The lightmay comprise a modulated optical signal having a modulation patterncorresponding to data to be transmitted. The modulated optical signalhas a wavelength component corresponding with a wavelength assigned tothe node 100. For example, the modulated output signal may be visible,infrared, or near-infrared light, ultraviolet, or a combination thereof.In some embodiments, the modulated output signal may have a wavelengthrange selected to satisfy existing WDM standards, such as CoarseWavelength Division Multiplexing (CWDM) or Dense Wavelength DivisionMultiplexing (DWDM). In some embodiments, the modulated output signalmay have a wavelength range selected to be compatible with other nodesof the avionics network. For example, each node in the network may beassigned a different wavelength.

The node 100 is arranged to receive, via the input 104, light from thenetwork. The light may comprise plural wavelength components. Forexample, the plural wavelength components may be wavelengths, or rangesof wavelengths, associated with or assigned to respective differentnodes of the network. Each of the wavelength components may therebycomprise an optical signal transmitted by another node of the network.

The node 100 comprises a wavelength selective filter 106. The wavelengthselective 106 is optically connected to the input 104. The wavelengthselective filter 106 is arranged to extract a wavelength component fromthe plural wavelength components of the light received via the input104. The wavelength selective filter 106 is arranged to extract awavelength component corresponding with the wavelength assigned to thenode 100. For example, the wavelength selective filter 106 may reflect awavelength component corresponding with the wavelength assigned to thenode 100.

The wavelength selective filter 106 thereby provides a first opticalsignal comprising the wavelength component corresponding with thewavelength assigned to the node 100 and a second optical signalcomprising a remainder of the plural wavelength components present inthe light received via the input 104.

The node 100 is arranged to transmit the second optical signal via theoutput 102.

In use, the node 100 is optically connected to a unit for transmittingand receiving optical signals, referred to hereinafter by way of exampleas an avionics unit 108. For example, the node 100 may comprise one ormore optical connectors comprising optical interfaces for interfacingwith corresponding interfaces of the avionics unit 108.

The wavelength selective filter 106 comprises two ports: an output port106 a and an input port 106 b. The output port 106 a is opticallyconnected to the output 102 and the input port 106 b is opticallyconnected to the input 104.

In the embodiment shown in FIG. 1, the node 100 comprises three opticalinterfaces for connecting to the avionics unit 108: a transmit interface110, a receive interface 112, and a monitor interface 114.

The node 100 comprises a first optical circulator 116, a second opticalcirculator 118, and a beam splitter 120. The first and second opticalcirculators 116, 118 each comprise a first port 122, a second port 124,and a third port 126. An optical circulator is a non-reciprocal,directional device that may comprise three or more ports for receivingand transmitting light. Light entering an optical circulator in any oneof the port exits the optical circulator only from the next port in asequential manner.

The first port 122 of the first optical circulator 116 is opticallyconnected to the transmit interface 110, the second port 124 of thefirst optical circulator 116 is optically connected to the output port106 a of the wavelength selective filter 106, and the third port 126 ofthe first optical circulator 116 is optically connected to the output102 via the beam splitter 120.

The first port 122 of the second optical circulator 118 is opticallyconnected to the input 104, the second port 124 of the second opticalcirculator 118 is optically connected to the input port 106 b of thewavelength selective filter 106, and the third port 126 of the secondoptical circulator 118 is optically connected to the monitor interface114.

The beam splitter 120 is arranged to receive light from the third port126 of the first optical circulator 116 and transmit light to the output102 and to the receive interface 112.

The avionics unit 108 comprises a transmitter 128, a receiver 130, and amonitor 132. However, it will be understood that in some embodiments themonitor 132 may be omitted. For example, the monitor 132 may be replacedby a non-reflective termination in the node 100 or in the avionics unit108.

In use, a first optical signal having a modulation pattern correspondingto data to be transmitted may be transmitted by the transmitter 128. Forexample, the transmitter 128 may comprise a light emitter (not shown)which emits light having a narrow range of wavelengths correspondingwith a wavelength assigned to the node 100. However, in someembodiments, the light emitter of the transmitter 128 may emit lighthaving a broad range of wavelengths components including a wavelengthcomponent corresponding with a wavelength assigned to the node 100.

The first optical signal transmitted by the transmitter 128 is receivedat the first port 122 of the first optical circulator 116 andtransmitted from the second port 124 of the first optical circulator 114to the output port 106 a of the wavelength selective filter 106.

The wavelength selective filter 108 is tuned to reflect a narrow rangeof wavelengths corresponding with the wavelength assigned to the node100 and to transmit other wavelength components.

The first optical signal is reflected by the wavelength selective filter106 back to the second port 124 of the first optical circulator 116. Thefirst optical signal is then transmitted from the third port 126 of thefirst optical circulator 116 to the beam splitter 120.

Wavelength components other than the wavelength component thatcorresponds with the wavelength assigned to the node 100 are transmittedvia the input port 106 b of the wavelength selective filter 106 to thesecond port 124 of the second optical circulator 118. These otherwavelength components are transmitted from the third port 126 of thesecond optical circulator 118 to the monitor interface 114, and are nottransmitted to other nodes of the network. In some embodiments, theseother wavelength components may be detected by a monitoring detector 132in the avionics unit 108, so that the avionics unit 108 can performmonitoring functions such as checking that the appropriate wavelengthcomponents are not present in the first optical signal, that the datamodulated on the optical signal is correct, or that the opticaltransmitter is performing as expected. In other embodiments, the otherwavelength components may be removed without detection by, for example,a non-reflective termination in the node 100 or the avionics unit 108.

The beam splitter 120 splits the first optical signal and directs thesplit first optical signal to the output 102, via which the firstoptical signal is transmitted to other nodes in the network, and to thereceive interface 112, via which the avionics unit 108 may receive thefirst optical signal to check that the signal has successfully beentransmitted.

Upon completion of transmission of the first optical signal through thenetwork, the first optical signal is received at the input 104.Simultaneously, optical signals having other wavelength components,transmitted by other nodes of the network, may be received at the input104.

Light received at the input 104 is transmitted to the first port 122 ofthe second optical circulator 118. The light is then transmitted fromthe second port 124 of the second optical circulator 118 to the inputport 106 b of the wavelength selective filter 106.

As explained above, the wavelength selective filter 106 is arrangedreflect a narrow range of wavelengths corresponding with the wavelengthassigned to the node 100 and to transmit other wavelength components.Therefore, the first optical signal is reflected back to the second port124 of the second optical circulator 118 and transmitted from the thirdport 126 of the second optical circulator 118 to the monitor interface114. In some embodiments, first optical signal may be detected by themonitoring detector 132, so that the avionics unit 108 can check thatthe first optical signal has successfully been received back at the node100 and can thereby detect faults in the network. In other embodiments,the first optical signal may be removed without detection by, forexample, a non-reflective termination in the node 100 or the avionicsunit 108.

Wavelength components present in the light received at the input 104other than the wavelength component to which the wavelength selectivefilter 106 is tuned (i.e. other than the wavelength assigned to the node100) are transmitted from the output port 106 a of the wavelengthselective filter 106 to the second port 124 of the first opticalcirculator 116 and then transmitted from the third port 126 of the firstoptical circulator 116 to the beam splitter 120. Those wavelengthcomponents are split by the beam splitter 120 and the split light isdirected to the output 102, via which it is transmitted to other nodesin the network, and to the receive interface 112, via which the avionicsunit 108 may receive those wavelength components to receive signals fromother nodes of the network.

FIG. 2 is a schematic diagram illustrating an example of a node 200 foran optical network according to another embodiment of the invention.Similarly to the node 100 shown in FIG. 1, the node 200 shown in FIG. 2is arranged to transmit an optical signal to, and receive opticalsignals from, other nodes in an optical network such as an opticalintegrated avionics network. It will be understood that components shownin FIG. 2 are shown symbolically and as such the symbols used do notnecessarily reflect the structure of the components they represent.

Similarly to the node 100 described with reference to FIG. 1, the node200 shown in FIG. 2 comprises an output 102, an input 104 and awavelength selective filter 106 comprising an output port 106 a and aninput port 106 b. The node 200 shown in FIG. 2 also comprises threeoptical interfaces for connecting to an avionics unit 108: a transmitinterface 110, a receive interface 112, and a monitor interface 114,similar to the node 100 described with reference to FIG. 1.

The node 200 comprises a first optical circulator 202, a second opticalcirculator 204, a third optical circulator 206 and a beam splitter 208.The first and third optical circulators 202, 206 each comprise a firstport 210, a second port 212, and a third port 214. The second opticalcirculator 204 comprises a first port 210, a second port 212, a thirdport 214 and a fourth port 216.

The first port 210 of the first optical circulator 202 is opticallyconnected to the transmit interface 110, the second port 212 of thefirst optical circulator 202 is optically connected to the output port106 a of the wavelength selective filter 106, and the third port 214 ofthe first optical circulator 202 is optically connected to the firstport 210 of the second optical circulator 204.

The second port 212 of the second optical circulator 204 is opticallyconnected to the first port 210 of the third optical circulator 206, thethird port 214 of the second optical circulator 204 is opticallyconnected to the input port 106 b of the wavelength selective filter106, and the fourth port 216 of the second optical circulator 204 isoptically connected to the monitor interface 114.

The second port 212 of the third optical circulator 206 is opticallyconnected to the output 102 via the beam splitter 208 and the third port214 of the third optical circulator 206 is optically connected to theinput 104. The beam splitter 208 is arranged to receive light from thesecond port 212 of the third optical circulator 206 and transmit lightto the output 102 and to the receive interface 112.

In use, the node 200 is optically connected to an avionics unit 108similarly to the node 100 described with reference to FIG. 1. A firstoptical signal having a modulation pattern corresponding to data to betransmitted may be transmitted by the transmitter 128. The first opticalsignal transmitted by the transmitter 128 is received at the first port210 of the first optical circulator 202 and transmitted from the secondport 212 of the first optical circulator 202 to the output port 106 a ofthe wavelength selective filter 106.

The first optical signal is reflected by the wavelength selective filter106 back to the second port 212 of the first optical circulator 202.Similarly to the embodiment described with reference to FIG. 1,wavelength components other than the wavelength component assigned tothe node 200 are transmitted via the input port 106 b of the wavelengthselective filter 106 to the third port 214 of the second opticalcirculator 204. These other wavelength components are transmitted fromthe fourth port 216 of the second optical circulator 204 to the monitorinterface 114, and are not transmitted to other nodes of the network. Insome embodiments, these other wavelength components may be detected bythe monitoring detector 132 in the avionics unit 108, so that theavionics unit 108 can check that inappropriate wavelength components arenot present in the first optical signal. In other embodiments, the otherwavelength components may be removed without detection by, for example,a non-reflective termination.

The reflected first optical signal is transmitted from the third port214 of the first optical circulator 202 to the first port 210 of thesecond optical circulator 204. The first optical signal is thentransmitted from the second port 212 of the second optical circulator204 to the first port 210 of the third optical circulator 206 and thentransmitted from the second port 212 of the third optical circulator 206to the output 102 via the beam splitter 208.

The beam splitter 208 splits first optical signal and directs the splitfirst optical signal to the output 102, via which the first opticalsignal is transmitted to other nodes in the network, and to the receiveinterface 112, via which the avionics unit 108 may receive the firstoptical signal to check that the signal has successfully beentransmitted.

Upon completion of the transmission through the network, the firstoptical signal is received at the input 104. Simultaneously, opticalsignals having other wavelength components transmitted by other nodes ofthe network may be received at the input 104.

Light received at the input 104 is transmitted to the third port 214 ofthe third optical circulator 206. The light is then transmitted from thefirst port 210 of the third optical circulator 206 to the second port212 of the second optical circulator 204 and from the third port 214 ofthe second optical circulator 204 to the input port 106 b of thewavelength selective filter 106.

The first optical signal is reflected back to the third port 214 of thesecond optical circulator 204 and transmitted from the fourth port 216of the second optical circulator 204 to the monitor interface 114. Insome embodiments, first optical signal may be detected by the monitoringdetector 132 in the avionics unit 108, so that the avionics unit 108 cancheck that the first optical signal has successfully been received backat the node 100 and can thereby detect faults in the network. In otherembodiments, the first optical signal may be removed without detectionby, for example, a non-reflective termination in the node 200 or theavionics unit 108.

Wavelength components present in the light received at the input 104other than the wavelength component to which the wavelength selectivefilter 106 is tuned (i.e. other than the wavelength assigned to the node100) are transmitted from the output port 106 a of the wavelengthselective filter 106 to the second port 212 of the first opticalcirculator 202, from the third port 214 of the first optical circulator202 to the first port 210 of the second optical circulator 204, from thesecond port 212 of the second optical circulator 204 to the first port210 of the third optical circulator 206, and from the second port 212 ofthe third optical circulator 206 to the beam splitter 208. Thosewavelength components are split by the beam splitter 120 and the splitlight is directed to the output 102, via which they are transmitted toother nodes in the network, and to the receive interface 112, via whichthe avionics unit 108 may receive those wavelength components to receivesignals from other nodes of the network.

In some embodiments, the node 200 may additionally comprise a removableconnector 218 via which the second port 212 of the second opticalcirculator 204 is optically connected to the first port 210 of the thirdoptical circulator 206. For example, the removable connector 218 maycomprise an expanded beam connector. The removable connector 218 enablesa portion of the node 200 comprising the wavelength selective filter 106to be removable and replaceable. This in turn enables the wavelength atwhich the node 200 is tuned to be changed. For example, the wavelengthselective filter 106 can be replaced with a wavelength selective filtertuned to a different wavelength to match the wavelength of the firstoptical signal.

FIG. 3 is a schematic diagram illustrating an example of a node 300 foran optical network according to another embodiment of the invention.Similarly to the node 100 shown in FIG. 1 and the node 200 shown in FIG.2, the node 300 is arranged to transmit an optical signal to, andreceive optical signals from, other nodes in an optical network such asan optical integrated avionics network. It will be understood thatcomponents shown in FIG. 3 are shown symbolically and as such thesymbols used do not necessarily reflect the structure of the componentsthey represent.

Similarly to the node 200 described with reference to FIG. 2, the node300 shown in FIG. 3 comprises an output 102, an input 104 and awavelength selective filter 106 comprising an output port 106 a and aninput port 106 b. The node 300 shown in FIG. 3 also comprises threeoptical interfaces for connecting to the avionics unit 108: a transmitinterface 110, a receive interface 112, and a monitor interface 114,similar to the node 100 described with reference to FIG. 1. Similarly tothe node 200 shown in FIG. 2, the node 300 also comprises a firstoptical circulator 202, a second optical circulator 204, a third opticalcirculator 206 and a beam splitter 208. The first and third opticalcirculators 202, 206 each comprise a first port 210, a second port 212,and a third port 214. The second optical circulator 204 comprises afirst port 210, a second port 212, a third port 214 and a fourth port216.

The first port 210 of the first optical circulator 202 is opticallyconnected to the transmit interface 110, the second port 212 of thefirst optical circulator 202 is optically connected to the output port106 a of the wavelength selective filter 106, and the third port 214 ofthe first optical circulator 202 is optically connected to the firstport 210 of the second optical circulator 204. However, in theembodiment shown in FIG. 3, the third port 214 of the first opticalcirculator 202 is optically connected to the first port 210 of thesecond optical circulator 204 via the beam splitter 208. The beamsplitter 208 is split light receive from the third port 214 of the firstoptical circulator 202 and direct the split light to the first port 210of the second optical circulator 204 and to the receive interface 112.

Similarly to the node 200 of FIG. 2, in the node 300 of this embodiment,the second port 212 of the second optical circulator 204 is opticallyconnected to the first port 210 of the third optical circulator 206, thethird port 214 of the second optical circulator 204 is opticallyconnected to the input port 106 b of the wavelength selective filter106, and the fourth port 216 of the second optical circulator 204 isoptically connected to the monitor interface 114.

Similarly to the node 200 of FIG. 2, in the node 300 of this embodiment,the second port 212 of the third optical circulator 206 is opticallyconnected to the output 102 and the third port 214 of the third opticalcirculator 206 is optically connected to the input 104.

In use, the node 200 is optically connected to an avionics unit 108similarly to the node 100 described with reference to FIG. 1. A firstoptical signal having a modulation pattern corresponding to data to betransmitted may be transmitted by the transmitter 128. The first opticalsignal transmitted by the transmitter 128 is received at the first port210 of the first optical circulator 202 and transmitted from the secondport 212 of the first optical circulator 202 to the output port 106 a ofthe wavelength selective filter 106.

The first optical signal is reflected by the wavelength selective filter106 back to the second port 212 of the first optical circulator 202.Similarly, to the embodiment described with reference to FIG. 1,wavelength components other than the wavelength component assigned tothe node 300 are transmitted via the input port 106 b of the wavelengthselective filter 106 to the third port 214 of the second opticalcirculator 204. These other wavelength components are transmitted fromthe fourth port 216 of the second optical circulator 204 to the monitorinterface 114, and are not transmitted to other nodes of the network. Insome embodiments, these other wavelength components may be detected bythe monitoring detector 132 in the avionics unit 108, so that theavionics unit 108 can check that inappropriate wavelength components arenot present in the first optical signal. In other embodiments, the otherwavelength components may be removed without detection by, for example,a non-reflective termination.

The reflected first optical signal is transmitted from the third port214 of the first optical circulator 202 to the beam splitter 208. Thebeam splitter splits the first optical signal and directs the splitfirst optical signal to the receive interface 112, via which theavionics unit 108 may receive those wavelength components to receivesignals from other nodes of the network, and to the first port 210 ofthe second optical circulator 204. The first optical signal is thentransmitted from the second port 212 of the second optical circulator204 to the first port 210 of the third optical circulator 206 and thentransmitted from the second port 212 of the third optical circulator 206to the output 102 to be transmitted to other nodes in the network.

Upon completion of the transmission through the network, the firstoptical signal is received at the input 104. Simultaneously, opticalsignals having other wavelength components transmitted by other nodes ofthe network may be received at the input 104.

Light received at the input 104 is transmitted to the third port 214 ofthe third optical circulator 206. The light is then transmitted from thefirst port 210 of the third optical circulator 206 to the second port212 of the second optical circulator 204 and from the third port 214 ofthe second optical circulator 204 to the input port 106 b of thewavelength selective filter 106.

The first optical signal is reflected back to the third port 214 of thesecond optical circulator 204 and transmitted from the fourth port 216of the second optical circulator 204 to the monitor interface 114. Insome embodiments, first optical signal may be detected by the monitoringdetector 132 in the avionics unit 108, so that the avionics unit 108 cancheck that the first optical signal has successfully been received backat the node 100 and can thereby detect faults in the network. In otherembodiments, the first optical signal may be removed without detectionby, for example, a non-reflective termination.

Wavelength components present in the light received at the input 104other than the wavelength component to which the wavelength selectivefilter 106 is tuned (i.e. other than the wavelength assigned to the node100) are transmitted from the output port 106 a of the wavelengthselective filter 106 to the second port 212 of the first opticalcirculator 202, and from the third port 214 of the first opticalcirculator 202 to the beam splitter 208. The beam splitter splits theremaining light and directs the split light to the receive interface112, via which the avionics unit 108 may receive signals from othernodes, and to the first port 210 of the second optical circulator 204,via which (and via the third optical circulator 206) the remaining lightmay be transmitted via the output 102 to other nodes of the network.

In this embodiment, there is no removable connector between the secondoptical circulator 204 and the third optical circulator 206, whichprovides a more efficient optical connection. However, it will beunderstood that a removable connector, such as the removable connector218 described with reference to FIG. 2, could be included.

In each of the nodes 100, 200, 300 described above with reference toFIGS. 1 to 3, optical signals of a wavelength assigned to the node 100,200, 300 can be emitted by the node 100, 200, 300, transmitted via thenetwork to other nodes, and removed by the node 100, 200, 300 havingbeen transmitted to the other nodes. This prevents signals transmittedby the node 100, 200, 300 from propagating through the networkindefinitely. Removal of the signal is done using passive componentsthat do not utilise high complexity configurable resources which aredifficult to test (as may be the case where wavelength selectiveswitches are used, for example) and which do not add latency at eachnode. In the embodiments described with reference to FIGS. 1 to 3, thisis achieved using a single wavelength selective filter that tunes theoutgoing signal to correspond with a wavelength assigned to the node,and then filters the returning incoming signal. This enables the node tobe operated with a generic avionics unit that utilises a broad-bandlight emitter. Using only a single wavelength selective filter ensuresthat the node only removes its own wavelength component.

FIG. 4 is a schematic diagram illustrating an example of a node 400 foran optical network according to another embodiment of the invention.Similarly to the nodes 100, 200, 300 shown in FIGS. 1, 2 and 3, the node400 is arranged to transmit an optical signal to, and receive opticalsignals from, other nodes in an optical network such as an opticalintegrated avionics network. It will be understood that components shownin FIG. 4 are shown symbolically and as such the symbols used do notnecessarily reflect the structure of the components they represent.

Similarly to the node 100 described with reference to FIG. 1, the node400 shown in FIG. 4 comprises an output 102, an input 104 and a firstwavelength selective filter 106 comprising an output port 106 a and aninput port 106 b similar to the wavelength selective filter describedabove with reference to FIGS. 1 to 3. The node 400 shown in FIG. 4 alsocomprises three optical interfaces for connecting to the avionics unit108: a transmit interface 110, a receive interface 112, and a monitorinterface 114, similar to the node 100 described with reference to FIG.1.

The node 400 comprises a first optical circulator 202 and a secondoptical circulator 204. The first optical circulator 202 comprises afirst port 210, a second port 212, and a third port 214. The secondoptical circulator 204 comprises a first port 210, a second port 212, athird port 214 and a fourth port 216.

The node 400 also comprises a first beam splitter 208 a and a secondbeam splitter 208 b.

The first port 210 of the first optical circulator 202 is opticallyconnected to the input 104, the second port 212 of the first opticalcirculator 202 is optically connected to the first port 210 of thesecond optical circulator 204, and the third port 214 of the firstoptical circulator 202 is optically connected to the output 102 via thefirst and second beam splitters 208 a, 208 b.

The second port 212 of the second optical circulator 204 is opticallyconnected to the input port 106 b of the first wavelength selectivefilter 106, the third port 214 of the second optical circulator 204 isoptically connected to the monitor interface 114, and the fourth port216 of the second optical circulator 204 is optically connected to theoutput port 106 a of the first wavelength selective filter 106.

The first beam splitter 208 a is arranged to receive light from thethird port 214 of the first optical circulator 202 and to transmit lightto the receive interface 112 and to the second beam splitter 208 b.

The second beam splitter 208 b, acts as a beam combiner and is arrangedto receive light from the transmit interface 110 and from the first beamsplitter 208 a and to transmit light to the output 102.

In use, the node 400 is optically connected to an avionics unit 108similarly to the node 100 described with reference to FIG. 1. A firstoptical signal having a modulation pattern corresponding to data to betransmitted may be transmitted by the transmitter 128. The first opticalsignal transmitted by the transmitter 128 is received at the second beamsplitter 308 b and directed to the output 102.

In embodiments in which the avionics unit is capable of providing asingle wavelength component itself, the first optical signal may betransmitted directly to the second beam splitter 208 b without furtherfiltering. However, in some embodiments, as shown in FIG. 4, the node400 comprises a third optical circulator 206 and a key interface 402arranged to receive a second wavelength selective filter 404. The thirdoptical circulator 206 comprises a first port 210, a second port 212 anda third port 214.

The first port 210 of the third optical circulator 206 is opticallyconnected to the transmit interface 110, the second port 212 of thethird optical circulator 206 is optically connected to the key interface402, and the third port 214 of the third optical circulator 206 isoptically connected to second beam splitter 208 b.

In use, a first optical signal having a modulation pattern correspondingto data to be transmitted may be transmitted by the transmitter 128. Forexample, the transmitter 128 may comprise a light emitter (not shown)which emits light having a broad range of wavelengths componentsincluding a wavelength component corresponding with a wavelengthassigned to the node 400. However, in some embodiments, the lightemitter of the transmitter 128 may emit light having a narrow range ofwavelengths corresponding with a wavelength assigned to the node 400.

The first optical signal transmitted by the transmitter 128 is receivedat the first port 210 of the third optical circulator 206 andtransmitted from the second port 212 of the third optical circulator 206to the key interface 402.

Light received at the key interface 402 is filtered by the secondwavelength selective filter 404. The second wavelength selective filter404 is tuned to reflect a narrow range of wavelengths corresponding withthe wavelength assigned to the node 400 and to transmit other wavelengthcomponents.

The first optical signal is reflected by the second wavelength selectivefilter 404 back to the second port 212 of the third optical circulator206. The first optical signal is then transmitted from the third port214 of the third optical circulator 206 to the second beam splitter 208b.

Wavelength components other than the wavelength component thatcorresponds with the wavelength assigned to the node are transmitted bythe second wavelength selective filter 404 and are removed without beingtransmitted to other nodes of the network. For example, the otherwavelength components may be removed by a non-reflective termination406.

The second beam splitter 208 b directs the first optical signal to theoutput 102, via which the first optical signal is transmitted to othernodes in the network.

Upon completion of the transmission through the network, the firstoptical signal is received at the input 104. Simultaneously, opticalsignals having other wavelength components transmitted by other nodes ofthe network may be received at the input 104.

Light received at the input 104 is transmitted to the first port 210 ofthe first optical circulator 202. The light is then transmitted from thesecond port 212 of the first optical circulator 202 to the first port210 of the second optical circulator 204, and from the second port 212of the second optical circulator 204 to the input port 106 b of thefirst wavelength selective filter 106. The first optical signal isreflected back to the second port 212 of the second optical circulator204 and transmitted from the third port 214 of the second opticalcirculator 204 to the monitor interface 114. In some embodiments, firstoptical signal may be detected by the monitoring detector 132 in theavionics unit 108, so that the avionics unit 108 can check that thefirst optical signal has successfully been received back at the node 100and can thereby detect faults in the network. In other embodiments, thefirst optical signal may be removed without detection by, for example, anon-reflective termination in the node 200 or the avionics unit 108.

Wavelength components present in the light received at the input 104other than the wavelength component to which the first wavelengthselective filter 106 is tuned (i.e. other than the wavelength assignedto the node 100) are transmitted from the output port 106 a of the firstwavelength selective filter 106 to the fourth port 216 of the secondoptical circulator 204. Those wavelength components are transmitted fromthe first port 210 of the second optical circulator 204 to the secondport 212 of the first optical circulator 202, and from the third port214 of the first optical circulator 202 to the output 102 via the firstand second beam splitters 208 a, 208 b.

The first beam splitter 208 a the light comprising only wavelengthcomponents other than that of the node 400 and directs the split lightto the receive interface 112, via which the avionics unit 108 mayreceive those wavelength components to receive signals from other nodesof the network and to the output 102 via the second beam splitter 208 b,via which the light is transmitted to other nodes in the network.

The nodes 100, 200, 300, 400 described above are able to tune thewavelength of outgoing optical signals and as such it is possible forthe networked units that generate the optical signals to utilisebroadband light emitters. The use of such emitters allows identical orgeneric units to be connected to nodes having a wavelength selectivefilters 106 that are tuned to respective different wavelengths, so thatthe generic avionics units may be used with optical networks utilisingwavelength division multiplexing (WDM).

In critical applications, such as aircraft avionics, each type ofavionics unit may have to undergo significant levels of testing beforebeing approved for use. Such testing may involve significant levels ofcost. Furthermore, each type of avionics unit needs to be stocked atoperating locations where the avionics unit may need to be replaced.Therefore, a generic avionics unit that can be used in optical networksthat utilise WDM may reduce the cost of putting avionics units intoservice. Furthermore, such a generic avionics unit may reduce levels ofstock held at operating locations and/or may reduce the time taken torestore an optical network (and a vehicle in which the network operates)to service when maintenance is required.

FIG. 5 is a schematic diagram illustrating an example of an opticalnetwork 500 according to an embodiment of the invention. The opticalnetwork 500 comprises a plurality of optical nodes at least one of whichcorresponds with a node 100, 200, 300, 400 as described above withreference 1 to 4.

In this embodiment, the network 500 is a ring network. However, it willbe understood that the invention may be implemented in other networktopologies.

The network 500 comprises a plurality of nodes each of which is arrangedto transmit an optical signal to other nodes in the network. Each nodemay, for example, transmit signals having a different wavelengthaccording to a WDM protocol as described above.

In use, a first node 502 transmits a first optical signal comprising afirst wavelength component to other nodes in the network 500 in aclockwise direction. Each other node 504 receives and detects the firstsignal and passively retransmits the first signal to the next node inthe network 500. When the first signal arrives back at the first node502, the first node 502 removes the first signal so that it does notcontinue to propagate through the network 500.

Unfiltered wavelength components are transmitted by the first node toother nodes in the network 500.

Although in the embodiments described above with reference to FIGS. 1 to4, the nodes 100, 200, 300, 400 are described as being used to connectavionics units to the network, it will be understood that the nodes maybe used to connect other networked apparatus to the network.Furthermore, it will be understood that in some embodiments, the nodemay be used as a bridge connecting a node in one network to a node inanother network.

FIG. 6 is a flow diagram illustrating a method 600 of operating anoptical network according to an embodiment of the invention.

At block 602, a first signal is transmitted by a node of the network.The first signal comprises a first wavelength component.

At block 604, light comprising plural wavelength components, includingthe first wavelength component, is received at the same node.

At block 606, the first wavelength component is extracted from thelight, thereby providing a second optical signal comprising theremainder of the plural wavelength components.

At block 608, the second optical signal is transmitted by the node.

In the embodiments described above, the transmit, receive and monitorinterfaces 110, 112, 114 may comprise any interface suitable fortransmitting optical signals. For example, one or more of the opticalinterfaces may be optical input-output interfaces. In some embodiments,one or more of the transmit, receive and monitor interfaces 110, 112,114 may comprise an expanded beam connector.

In the embodiments described above, the receiver 130, where provided,may comprise a detector capable of detecting light. In some embodiments,the detector may be one of a photodiode, a phototransistor, an activepixel sensor, a charge-coupled device or a direct-conversion radiationdetector.

It will be understood that, although in some embodiments the wavelengthselective filter(s) connected to a node may be part of a wiring harness,in other examples the wavelength selective filter(s) may be separate tothe wiring harness. In some embodiments, the wavelength selectivefilter(s) may be supplied as keys arranged to plug into a correspondingport or optical interface of the node. The nodes may be supplied with orwithout the wavelength selective filter(s). In some embodiments, a nodeunit may be supplied with a wavelength selective filter connected to theremovable connector.

The wavelength selective filter 106 may be any filter suitable forfiltering a relatively narrowband signal from a broadband signal. Thewavelength selective filter(s) of embodiments of the invention may bearranged to extract the narrowband optical signal in transmission or inreflection. For example, the wavelength selective filter 106 maycomprise one of a fibre Bragg grating, a Long Period Grating, and aTilted Fibre Bragg grating. Such gratings may, for example, reflect anarrow range of wavelengths and transmit wavelengths outside of thenarrow band. In other examples, the wavelength selective filter 106 maycomprise a Fabry-Perot interferometer, a fibre interferometer, adiffraction grating, and a prism.

The avionics unit 108 described above may be, for example, a LineReplaceable Unit (LRU) or any other type of avionics unit. The network500 may be, for example, an Integrated Modular Avionics (IMA) network.

Referring to FIG. 7, there is shown a schematic side view of an exampleof a vehicle according to an embodiment of the invention. In the exampleof FIG. 7, the vehicle is an aircraft 700. The aircraft 700 may compriseone or more networks, such as the network 500 described above withreference to FIG. 5. The, or each, network 500 may comprise one or morenodes, such as the nodes 100, 200, 300, 400 described above withreference to FIGS. 1 to 4. In other embodiments, the vehicle may beother than an aircraft, such as a road vehicle, a rail vehicle, awatercraft or a spacecraft.

The above embodiments are to be understood as illustrative examples ofthe invention. It is to be understood that any feature described inrelation to any one embodiment may be used alone, or in combination withother features described, and may also be used in combination with oneor more features of any other of the embodiments, or any combination ofany other of the embodiments. Furthermore, equivalents and modificationsnot described above may also be employed without departing from thescope of the invention, which is defined in the accompanying claims.

1. A node for an optical network, the node comprising: an input toreceive from an optical network light comprising plural wavelengthcomponents; an optical wavelength selective filter, optically connectedto the input, to extract a first wavelength component of the pluralwavelength components from the light, thereby providing a first opticalsignal comprising the first wavelength component and a second opticalsignal comprising a remainder of the plural wavelength components; afirst output optically connected to the optical wavelength selectivefilter to receive a first portion of the second optical signal fortransmission to a light detector; and a second output opticallyconnected to optical wavelength selective filter to receive a secondportion of the second optical signal for transmission to the opticalnetwork.
 2. The node according to claim 1, comprising one or moreoptical connectors to connect the node to an avionics unit, the one ormore optical connectors comprising: a first optical interface, opticallyconnected to the second output, to receive an optical signal from theavionics unit; a second optical interface, optically connected to thefirst output, to transmit the first portion of the second optical signalto the avionics unit; and a third optical interface, optically connectedto the optical wavelength selective filter, to transmit the firstoptical signal to the avionics unit.
 3. The node according to claim 2,comprising a first optical circulator, a second optical circulator, anda beam splitter, wherein the first and second optical circulators eachcomprise a first port, a second port, and a third port, and the firstoptical wavelength selective filter comprises a first port and a secondport, wherein: the first port of the first optical circulator isoptically connected to the first optical interface, the second port ofthe first optical circulator is optically connected to the first port ofthe first optical wavelength selective filter, and the third port of thefirst optical circulator is optically connected to the output via thebeam splitter; the first port of the second optical circulator isoptically connected to the input, the second port of the second opticalcirculator is optically connected to the second port of the firstoptical wavelength selective filter, and the third port of the secondoptical circulator is optically connected to the third opticalinterface; wherein the beam splitter is arranged to receive light fromthe third port of the first optical circulator and transmit light to theoutput and to the second optical interface.
 4. The node according toclaim 2, comprising a first optical circulator, a second opticalcirculator, a third optical circulator and a beam splitter, wherein thefirst and third optical circulators each comprise a first port, a secondport, and a third port, the second optical circulator comprises a firstport, a second port, a third port and a fourth port, and the firstoptical wavelength selective filter comprises a first port and a secondport, wherein: the first port of the first optical circulator isoptically connected to the first optical interface, the second port ofthe first optical circulator is optically connected to the first port ofthe first optical wavelength selective filter, and the third port of thefirst optical circulator is optically connected to the first port of thesecond optical circulator; the second port of the second opticalcirculator is optically connected to the first port of the third opticalcirculator, the third port of the second optical circulator is opticallyconnected to optically connected to the second port of the first opticalwavelength selective filter, and the fourth port of the second opticalcirculator is optically connected to the third optical interface; thesecond port of the third optical circulator is optically connected tothe output via the beam splitter and the third port of the third opticalcirculator is optically connected to the input; wherein the beamsplitter is arranged to receive light from the second port of the thirdoptical circulator and transmit light to the output and to the secondoptical interface.
 5. The node according to claim 4, wherein the secondoptical circulator is optically connected to the third opticalcirculator via a removable connector.
 6. (canceled)
 7. The nodeaccording to claim 2, comprising a first optical circulator, a secondoptical circulator, a third optical circulator and a beam splitter,wherein the first and third optical circulators each comprise a firstport, a second port, and a third port, the second optical circulatorcomprises a first port, a second port, a third port and a fourth port,and the optical wavelength selective filter comprises a first port and asecond port, wherein: the first port of the first optical circulator isoptically connected to the first optical interface, the second port ofthe first optical circulator is optically connected to the first port ofthe first optical wavelength selective filter, and the third port of thefirst optical circulator is optically connected to the first port of thesecond optical circulator via the beam splitter; the second port of thesecond optical circulator is optically connected to the first port ofthe third optical circulator, the third port of the second opticalcirculator is optically connected to optically connected to the secondport of the first optical wavelength selective filter, and the fourthport of the second optical circulator is optically connected to thethird optical interface; the second port of the third optical circulatoris optically connected to the output and the third port of the thirdoptical circulator is optically connected to the input; wherein the beamsplitter is arranged to receive light from the third port of the firstoptical circulator and transmit light to the first port of the secondoptical circulator and to the second optical interface.
 8. The nodeaccording to claim 2, comprising a first optical circulator, a secondoptical circulator, a first beam splitter and a second beam splitter,wherein the first and third optical circulators each comprise a firstport, a second port, and a third port, the second optical circulatorcomprises a first port, a second port, a third port and a fourth port,and the optical wavelength selective filter comprises a first port and asecond port, wherein: the first port of the first optical circulator isoptically connected to the input, the second port of the first opticalcirculator is optically connected to the first port of the secondoptical circulator, and the third port of the first optical circulatoris optically connected to output via the first and second beamsplitters; the second port of the second optical circulator is opticallyconnected to the first port of the first optical wavelength selectivefilter, the second port of the second optical circulator is opticallyconnected to the third optical interface, and the fourth port of thesecond optical circulator is optically connected to the second port ofthe first optical wavelength selective filter; the first beam splitteris arranged to receive light from the third port of the first opticalcirculator and to transmit light to the second optical interface and tothe second beam splitter; and the second beam splitter is arranged toreceive light from the first optical interface and from the first beamsplitter and to transmit light to the output.
 9. The node according toclaim 8, wherein the first optical circulator is optically connected tothe second optical circulator via a removable connector.
 10. (canceled)11. The node according to claim 8, comprising a third optical circulatorand a fourth optical interface arranged to receive a removable secondoptical wavelength selective filter, the third optical circulatorcomprising a first port, a second port and a third port, wherein: thefirst port of the third optical circulator is optically connected to thefirst optical interface, the second port of the third optical circulatoris optically connected to the fourth optical interface, and the thirdport of the third optical circulator is optically connected to secondbeam splitter.
 12. (canceled)
 13. The node according to claim 11,comprising the second optical wavelength selective filter, the secondoptical wavelength selective filter having a same wavelength selectivitysubstantially matching a wavelength selectivity of the first opticalwavelength selective filter. 14.-15. (canceled)
 16. The node accordingto claim 2, comprising an avionics unit, the avionics unit comprising: alight emitter to provide a modulated broadband optical signal to thefirst optical interface; and a first light detector to receive lightfrom the second optical interface.
 17. The node according to claim 16,wherein the avionics unit comprises a second light detector arranged toreceive light from the third optical interface.
 18. The node accordingto claim 16, comprising a non-reflective termination optically connectedto the third optical interface.
 19. (canceled)
 20. A node for an opticalnetwork, the node comprising: an output to transmit to an opticalnetwork a first optical signal comprising a first wavelength component;an input to receive, from the optical network, light comprising pluralwavelength components; and an optical interface optically connected tothe input and the output, the first optical interface being to receive aremovable optical wavelength selective filter arranged to extract thefirst wavelength component of the plural wavelength components from thelight, thereby providing an optical signal comprising the firstwavelength component and a second optical signal comprising a remainderof the plural wavelength components.
 21. A system, comprising: a nodeaccording to claim 20; and a removable optical wavelength selectivefilter to optically connect to the optical interface of the node, theoptical wavelength selective filter being to extract the firstwavelength component of the plural wavelength components from the light.22. The system according to claim 21, wherein the optical wavelengthselective filter is optically connected to the optical interface of thenode.
 23. (canceled)
 24. The system according to claim 21, wherein theremovable optical wavelength selective filter comprises a non-reflectivetermination. 25.-26. (canceled)
 27. An optical network comprising pluralnodes, each of the plural nodes being arranged to transmit an opticalsignal at a distinct wavelength, wherein a node of the plural nodes isarranged to: transmit a first optical signal comprising a firstwavelength component; receive light comprising plural wavelengthcomponents including the first wavelength component; extract the firstwavelength component from the light, thereby providing a second opticalsignal comprising a remainder of the plural wavelength components; andtransmit the second optical signal. 28.-29. (canceled)
 30. A method ofoperating a node, wherein the node comprises: an input to receive froman optical network light comprising plural wavelength components; anoptical wavelength selective filter, optically connected to the input,to extract a first wavelength component of the plural wavelengthcomponents from the light, thereby providing a first optical signalcomprising the first wavelength component and a second optical signalcomprising a remainder of the plural wavelength components; a firstoutput optically connected to the optical wavelength selective filter toreceive a first portion of the second optical signal for transmission toa light detector; and a second output optically connected to opticalwavelength selective filter to receive a second portion of the secondoptical signal for transmission to the optical network, wherein themethod comprises: transmitting an optical signal comprising the firstwavelength component from the node; receiving the light comprisingplural wavelength components including the first wavelength component atthe node; and extracting the first wavelength component from the lightat the node, thereby providing the second optical signal comprising theremainder of the plural wavelength components.
 31. (canceled)